Credits to EmagDrolBot for a solution. and I've added some extra things to make it quick.
I've just created a world for this achievement and you can find a link below.
All you need to do is...
Download the file from the link below.
To Import the World
1. From Main Menu --> Tools --> Scenario Editor. Once it's opened, click "File" (on the Top Left corner) then select "Load Scenario". (Click "NO" for saving the currently opened world because it's just a blank world, so click "no")
2. Now, Click "Open Saved Games" to open the folder that contains save files. Then Copy the downloaded file from the downloads folder to this(Folder that opened through AOE3) location.
3. Then close the File Explorer and back out from the Load Scenario to refresh that page. Open "Load Scenario" again and you can see the downloaded file appears on the list.
4. Now, you can open through "Scenario Editor" or from Main Menu --> Load --> Use Filter setting --> Custom Scenario --> "Save File" --> Open.
5. If you opened through the scenario editor, click scenario on the top bar, then select playtest scenario, make sure it selected as Player 1.
6. There will be plenty of Islands to convert the enemies in that saved world, so move Island by island, Select all priestesses on one island(10 priestesses per island), and click the "convert" button from the bottom menu or use "Q"(Shortcut key for convert) and click on the enemy to convert.
You'll get 10% on your progression for one island. so 10 Islands are enough to get the achievement.
The achievement will pop once you've completed conversion on 10 Islands.
Save File: https://drive.google.com/drive/folders/1FXZmB5Kk9Ogy2gLxqF8p...
You Can also find 2 other achievements save files inside the link.
Following Achievements are